The grubs sub for now is about 90% decomposing wood ground up, that was mixed with a tiny bit of pulverized leaf mold substrate I give most grubs, and then I compressed it into a loose block.
I have some substrate mixes that I am planning on trying out when they need to be moved into larger containers, more of the same, some "fermented" substrate, a couple blends of that sort, and I inoculated a couple containers of wood/rice flour mix with oyster mushroom mycelium the other day(my take on kinshi bottles).